While liquid crystal displays (LCDs) offer a compact, lightweight alternative to cathode ray tube (CRT) monitors, there are many applications for which LCDs are not satisfactory due to a low level of brightness, or more properly, luminance. The transmissive LCD that is used in known laptop computer displays is a type of backlit display, having a light-providing surface positioned behind the liquid crystal (LC) array for directing light outwards, towards the LCD. The light-providing surface itself provides illumination that is essentially Lambertian, having an essentially constant luminance over a broad range of angles.
With the goal of increasing on-axis and near-axis luminance, a number of brightness enhancement films have been proposed for redirecting a portion of this light having Lambertian distribution toward normal, relative to the display surface. There have been many proposed solutions for brightness or luminance enhancement for use with LCD displays and with other types of backlit display types.
U.S. Pat. No. 6,111,696 (Allen et al.) describes a brightness enhancement film for a display or lighting fixture. The surface of the optical film facing the illumination source is smooth and the opposite surface has a series of structures, such as triangular prisms, for redirecting the illumination angle. U.S. Pat. No. 5,629,784 (Abileah et al.) describes various embodiments in which a prism sheet is employed for enhancing brightness, contrast ratio, and color uniformity of an LCD display of the reflective type. The brightness enhancement film is arranged with its structured surface facing the source of reflected light for providing improved luminance as well as reduced ambient light effects. U.S. Pat. No. 6,356,391 (Gardiner et al.) describes a pair of optical turning films for redirecting light in an LCD display, using an array of prisms, where the prisms can have different dimensions.
U.S. Pat. No. 6,280,063 (Fong et al.) describes a brightness enhancement film with prism structures on one side of the film having blunted or rounded peaks. U.S. Pat. No. 6,277,471 (Tang) describes a brightness enhancement film having a plurality of generally triangular prism structures having curved facets. U.S. Pat. No. 5,917,664 (O'Neill et al.) describes a brightness enhancement film having “soft” cutoff angles in comparison with known film types, thereby mitigating the luminance change as viewing angle increases.
While known approaches, such as those noted above, provide some measure of brightness enhancement at low viewing angles, these approaches have certain shortcomings. Some of the solutions noted above are more effective for redistributing light over a preferred range of angles rather than for redirecting light toward the normal for best on-axis viewing. These brightness enhancement film solutions often exhibit a directional bias, working best for redirecting light in one direction. For example, a brightness enhancement film may redirect some of the light in the vertical direction to relatively high off-axis angles that is out of the desired viewing cone. In another approach, multiple orthogonally crossed sheets are overlaid in order to redirect light in different directions, typically in both the horizontal and vertical directions with respect to the display surface. Necessarily, this type of approach is somewhat of a compromise; such an approach is not optimal for light in directions diagonal to the two orthogonal axes. In addition, such known films typically use “recycling” in which the light is reflected back through the backlight module multiple times in an effort to increase brightness. However, some of the reflected light is absorbed by materials and lost in reflection during recycling.
As discussed above, brightness enhancement layers have been proposed with various types of refractive surface structures formed atop a substrate material, including arrangements employing a plurality of protruding prism shapes, both as matrices of separate prism structures and as elongated prism structures, with the apex of prisms both facing toward and facing away from the light source. For the most part, these films exhibit directional bias, with some of the light poorly directed.
Certain types of light redirecting layers rely on Total Internal Reflection (TIR) effects for redirecting light. When a light guide is employed and such features are included in contact with the light output surface of the light guide, the features are more correctly termed “light extracting” features since they enable the light to be output rather than simply redirecting existing light. If the light guide is surrounded by air, features are needed to extract the light. These layers include prism, parabolic or aspheric structures, which re-direct light using TIR. For example, U.S. Pat. No. 5,396,350 to Beeson et al., describes a backlight apparatus comprising a slab waveguide and an array of microprisms attached on one face of the slab waveguide. U.S. Pat. Nos. 5,739,931 and 5,598,281 to Zimmerman et al. describe illumination apparatus for backlighting, using arrays of microprisms and tapered optical structures. U.S. Pat. No. 5,761,355 to Kuper et al. describes arrays for use in area lighting applications, wherein guiding optical structures employ TIR to redirect light towards a preferred direction. U.S. Pat. No. 6,129,439 to Hou et al. describes an illumination apparatus in which microprisms utilize TIR for light redirection. Japanese Laid-open Patent Publication No. 8-221013 entitled “Plane Display Device And Backlight Device For The Plane Display Device” by Yano Tomoya (published 1996) describes an illumination apparatus having collimating curved facet projections for light redirection utilizing TIR. U.S. Pat. No. 6,425,675 to Onishi et al., using curved facets similar to those originally described in the Tomoya 8-221013 disclosure, describes an illumination apparatus in which a light output plate also has multiple curved facet projections with their respective tips held in tight contact with the light exit surface of a light guide member.
A number of patent disclosures, such as the Tomoya 8-221013 and '675 Onishi et al. disclosures cited above, employ films having projecting structures and specify that these structures have one or more curved surfaces. While the use of a curved surface for TIR may be useful for providing on-axis light extraction, the design of curved projections for obtaining light over a broader range of angles can be more difficult. Moreover, curved surfaces themselves can prove to be difficult to fabricate, particularly at the dimensional scale that is needed for structures of a light extracting film.
Light extracting films must be optically coupled to their corresponding light guiding component in some way. Embodiments using structures with flat light input surfaces can be optically coupled simply by physical contact with the light guide, provided that this contact is maintained. Embodiments using structures with curved light input surfaces must be held in tight contact against the light guide. In order to prevent the tips of the projections of the light output plate from being embedded in the bonding layer, the bonding agent is semi-hardened beforehand and, after the bonding layer and the tips of the projections are brought to a tight contact each other, the bonding agent is hardened completely, as noted in the Onishi et al. '675 disclosure; however, the use of a two step hardening process, as described, can increase cost and complexity of fabrication. Also described in the art is a method for stacking surface structured optical films in which the structured surface of one film is bonded to an opposing surface of second film using a layer of adhesive by penetrating the structured surface into the adhesive layer to a depth less than a feature height of the structured surface, see U.S. Pat. No. 6,846,089 and U.S. 2005/0134963 A1. This, however, does not provide for more effective light extraction from a light guide plate.
What is needed, therefore, is a light extracting film that overcomes at least the shortcomings of known films previously described and that can be fabricated at reasonable cost.